Voltage indicating circuit



Feb. 15, 1944.

'J. E. MAYNARD VOLTAGE INDICATING CIRCUIT Filed Feb. 8} 1945 Fig! TU/V/NG 0 ON TROL rum/vs IND/6A TOR Inventor. John E. Maynard, by

His Attorney.

Patented Feb. 15, 1944 VOLTAGE INDICATING CIRCUIT John E. Maynard, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application February 8, 1943, Serial No. 475,221

'7 Claims.

My invention relates to visual voltage indicating circuits and in particular to visual voltage. indicating circuits for use in the tuning of radio receivers.

It is an object of my invention to provide an improved visual voltage indicating circuit for use as a tuning meter for radio receivers.

In receivers for frequency modulated radio waves, it is customary to employ a balanced frequency discriminator circuit as a means to demodulate the modulated radio frequency waves and to reproduce therefrom the modulating signals. Since the modulating signals appear as frequency deviations above and below the average carrier frequency, in the tuning of the receiver it is desirable, in order to obtain optimum reproduction of the signals by the receiver, to adjust the mid point of the discriminators range of frequencies to match the average carrier frequency. Accordingly, it is an object of my invention to provide an improved tuning circuit by which the center frequency of a discriminator circuit may be adjusted to the proper value with ease and accuracy.

Another object of my invention is to provide an improved tuning indicator for frequency modulation receivers whichis rugged and relatively inexpensive and which is particularly adapted for use on portable receivers.

One of the features of my invention is the use of an electron-ray tube having a fluorescent screen and a pair of electrodes. The potential of one of the electrodes varies in accordance with a potential being calibrated, while the potential of the other electrode is maintained at a value cor.-

responding to the predetermined voltage to be indicated.

Another object of my invention is to provide an improved voltage indicating circuit employing an indicator of the cathode-ray type in whicha first shadow width is maintained at a value corresponding to a predetermined voltage and a secand shadow width varies in accordance with a voltage being adjusted.

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying, drawing; in which Fig. lrepresents a portion of the frequencymodulation receiver. circuit embodying my invention.

and Figs. 2-4 illustrate certain operating acteristics of my indicator circuit.

Referring to the drawing, I have shown an a ntenna l for receiving radio signals and supply: ing them to the radio frequency circuits 2 of a receiver. These circuits, which may be'of any desired type, are indicated merely by the rectangle labeled 2 and include means for tuning the re,- ceiver to desired frequencies within a range of frequencies in response to rotation of a tuning control knob 3. The radio frequency voltages, after amplification in the circuits 2, and conversion to a constant intermediate frequency,if the receiver be of the superheterodyne type, ar supplied to the tuned primary winding of the trans.- former of a frequency discriminator II, the functionoi the discriminator circuit being to demodulate the modulated radio frequency input waves and to reproduce therefrom the modulating signals, which may be voice or music, and to supply them by means of conductor l2 to subsequent stages of the receiver. The indicating circuit I3 is provided to indicate the desired tuning condition. of the receiver circuits. 7 V

The frequency discriminator circuit H is arranged to supply to the conductor I2 a potential which varies from a predetermined value, as for example ground potential, either positively or negatively depending upon the variations of the intermediate frequency from a'desir'ed fixed value. From the output of the discriminator, the signals are supplied by the lead I2 to further stageso amplification or to a sound reproducer.

The discriminator circuit H is of well-known form and comprises a transformer having a primary winding l4 and a secondary winding l'5,.both of these windings being tunable to' the desired fixed intermediate frequency by'means of capacitors l6 and ll. Operating potential for thepreceding stages may be supplied from asuitable source, such as the batter l8 through primary winding li which is connected to ground foralternating currents by means of by-pass capa'citor I9 and to the midpointof secondary winding l5 through av condenser 20. The .oppositeterminals of the secondary winding l5 are connected to the respective anodes of diodes 21 and 22',jthe cathodes of these diodes being connectedtogether for alternating currents through condenser 23 and for direct currents through resistances 2'4 and125. The cathodeiof diode 22' is grounded and the midpoint between resistances z l' and 25 islconnected'toi the. midpoint of secondary'winding lfi through' a chokefc'oil 2B. ln o'rder tb'minim'ize the. efi'ect of resistor 25 on the'quality'value Q f device 22.

--lation with the primary voltage.

In the desired operation of the discriminator circuit, when the tuning control of the radio receiver is adjusted for accurate resonance with the received carrier wave and the intermediate frequency has the value to which the primary and the secondary windings l4 and I5 are each tuned, the voltage across the secondary winding of the transformer, in accordance with wellknown theory, is displaced in phase from the voltage across the primary by 90. With the connection shown, the voltage of the primary is connected in series with one-half of the secondary across discharge device 2| and condenser 23, and the primary is connected in series with the other half of the'secondary across discharge Because of the quadrature relation between the primary and secondary voltages, the voltage on one-half of the secondary leads the voltage on the primary by 90, whereas that on the-other half of the secondary lags behind the voltage on the primary by 90. Thus, the voltages applied to the two diodes 2| and 22, when the intermediate frequency is at its desired value, are equal and accordingly equal values of unidirectional current flow through each of the diodes and hence through resistances 24 and 25. It will be observed that the diodes are so poled that the voltages across the two resistances 24 and 25 are opposite in polarity in the circuit between conductor l2 and ground with the result that conductor |2 is at ground potential when the intermediate frequency is at the desired value.

The quadrature relation between the primary and secondary voltages of the discriminator transformer exists, however, only when the oscillations supplied thereto have the desired intermediate frequency. If this frequency changes in either direction, the phase of the secondaryvoltage varies from its 90 relation with the primary voltage in one direction or the other, dependent upon whether the frequency increases or decreases. If the frequency increases, the phase shift may be in such a direction that the voltage on the upper half of the secondary winding approaches the aiding relation with the primary voltage, whereas that on the lower half of the secondary winding approaches the opposing re- Thus, the voltage applied to diode 2| increases and that applied to diode 22 decreases with the result that the unidirectional potential on resistance 24 increases, whereas that on resistance 25 decreases and the conductor |2 thus becomes positive with respect to ground. On the other hand, if theintermediate frequency decreases, an opposite shift in phase of the secondary voltage occurs with the result that the larger alternating current voltage is supplied to diode 22 and the potential on-resistance 25 increases and that on resistance 24 decreases and the conductor I2 is driven negative with respect to ground.

In order to determine that the receiver circuits are tuned effectively to the same frequency to which the windings of the discriminator circuit device. the sensitivity of the circuit depends upon the pentode operates as a triode amplifier and provides the advantages of a high transconductance offered by a pentode type of electron discharge or course, any triode may be used and transconductance of the particular triode used. The resistor 31"is of a high value, for example, of the order of 1 megohm, in order to avoid loading the input circuit of the amplifier by grid current in the device 39, as well as to prevent any, loading of the output circuits of .the discriminator.

The electron-ray tube 3| is a well-known twin type electric eye tube. It contains tWo control electrodes 38 and .39 for producing two independent angles on the fluorescent screen or anode 40 and acap 40', located in the tube approximately as shown in Fig. 1, and which causes the anode to appear as a luminous ring to a person viewing the indicating end of the tube. In. this type of tuning indicator, electrons travel radially outwardly from the cathode 4| to the fluorescent screen or anode 40 in a wide beam and the extent of the surface of the anode reached by the electron beam is determined by the voltage on the electrodes 38 and 39. The more positive these electrodes are with respect to cathode 4|, the greater is the area of the anode covered by the beam and the smaller the shadow angles cast by these electrodes. Operating potential for the anode 49 is provided from any suitable source, such as the battery 42, through a voltage dropping resistor 43 and the anode 40 of the electronray tube is connected to anode 33 of the amplifier 30 through a resistor 44 which functions as a load Resistor 44 is of large value, for example, 0.5 megohm, in order to provide a wide range of control voltage for the control electrode 38 of the electron-ray tube 3|.

A voltage divider, comprising resistors 45, 46, and 41, is connected in series with voltage dropping resistor 43 across the battery 42 in order to provide suitable bias voltages for the tubes 39 and 3|. Cathode 35 of the amplifier 30 is connected to an adjustable point on resistor 41 of this voltage divider and both control electrode 39 and cathode 4| of the electron-ray tube are connected to the common point of resistors 45 and 46. In some applications it may be desirable to connect electrode 39 to an intermediate point on either resistor 45 or resistor 45 in order to 'maintain electrode 39 at a fixed positive or negative potential with respect to cathode 4| and provide a desired calibration pattern.

In the adjustment and operation of the indicator circuit I3, the Voltage difference between cathode 4| and anode 40 of the electron-ray tube, that is, the unidirectional potential across resistor 45, determines the brightness of the indicator pattern and also aiiects the opening angle of the indicator patterns determined by electrodes 38 and 39. The indicator pattern determined by electrode 39 functions as and is hereinafter referred to as the calibrated or reference pattern. .The shadow angle determined by electrode 38,

whichis hereinafter called the indicator pattern,

is adjusted to show the same angle of opening as that shown onthecalibrated or reference pattern for apredetermined voltage to be indicated. In a frequency modulation receiver, when the in: dicator I3 is used in conjunction with the discriminator l I, the voltage to beindicated, that is, the potential of conductor I2 in the absence of a modulating signal in the receiver circuit, is-ground potential or zero voltage. In adjusting the indicator 13 so that the indicator angleor pattern produced by control electrode 38 matches that of the calibrated patternwhen there is zero voltage on conductor l2, control electrode 36 is connected directly to ground and the potential between control electrode 35 and cathode 35 is adjusted by means of the contact arm 48 on resistor ll so; that the same angle of opening is shown on both the indicator and calibrator patterns.

'When the above adjustment has been made, the indicator circuit is ready for operation: In put Voltages more positive than zero cause more current to flow in resistor 44 and make the con-' trol Voltage of electrode 38 less positive, opening the shadow angle of the indicator pattern. Con denser 49 is provided to remove audio frequency voltages from control electrode 36 and give an increased clearness of the indicator pattern. Input voltages on conductor [2 of negative polarity cause less current to flowin resistor 44 and make the control voltage of electrode 38 more positive than the calibrated electrode 39 and are effective to decrease the angle of the indicator pattern. In this way, in the tuning of the receiver, the crossover point of the discriminator output circuit, where the voltage on conductor l2 changes from positive to negative, and hence the'position of the tuning control knob at which the receiver is tuned precisely to the received carrier wave, may be determined withease and accuracy.

The operation of the indicator circuit maybe illustrated by Figs. 2-4. In Fig. 2, the desired condition of precise tuning, where the output voltage of the discriminator circuit is zero, is shown. The reference pattern 58 is held constant at an angle determined by the potential of electrode39 with respect to cathode M. The opening angle of the indicator pattern 51', determined by the potential of electrode 38 with respect to cathode 4|, under the desired condition of tuning is equal to the angle of the reference pattern. Fig. 3 illustrates the condition when the frequency of the voltage applied to thediscrimi nator circuits is higher than that to which these circuits are tuned. The output voltages of the discriminator is their positive and the shadow angle 52 of the" indicator pattern is larger than that of the reference pattern Fig. 4 illustrates the condition when the output voltage of the discriminator is negative and the" shadow angle 53 of the indicator pattern is smaller than that of the reference pattern 50.

The indicator thus disclosed provides a circuit and apparatus, particularly adapted for tuning frequency modulation receivers, which is more rugged and less expensive than instrument types. of indicators, such as-"micr'oammete'rsz' It isespe cially adapted for use in portable receivers where instruments are affected by vibration and similar shocks. Since a reference pattern is provided which indicatesadesired'tuning condition, the receiver may easily be adjusted so that the indicator pattern matches this reference pattern. While such accuracy of tuning is desirable in any t'ype'of radio receiver, whether being used* for the reception of broadcast-signals or other typesof signals, it is particularly desirable for tuningaccurately to the center frequency of awide band in a frequency modulation system or in any typeof receiver using a discriminator transformer:

Since the electron-ray tube 3i isoperatedwithits cathode considerably above ground potential, a wide range of potentials for electrode 38 is permitted so that wide angles of indication for the indicator pattern from zero to substantially 180 may be used and hence the maximum visualsensitivity of the indicator utilized. It is appar-' ent, of course, that the cathode of the device 3| may be operated at ground potential, such operation limiting the opening angles of the indicator pattern to approximatelyQO. Since the entire bias system for the tubes 30 and 3! is contained in the same network, voltage variations in this network affect all unidirectional tube potentials in the same manner and in the same percent so that calibration accuracy is maintained despite such variations.

In this .respect, it is noted that sensitivity of the circuit to variation of the input voltage is a function of the amplification of amplifier 39, the control sensitivity of the electron-ray tube 3i, and the visual judgment of the observer but has no relation to the total amount of voltage variation in the voltage to be indicated. By this is meant that an adjustment of the discriminator output voltage can be made to an accuracy of a definite amount of voltage variation from zero potential regardless of the input potential of the discriminator.

By way of illustration only and not in any sense by way of limitation, the following values are given for a particular frequency modulating receiver embodying my invention. In this receiver a type 68H? tube was employed as the amplifier 30 and a type GADG-G tube as the electron-ray tube 3i. When the value of resistor M was 0.5 megohm and of resistor 3i, 1 megohm, the bias potential between contact :18 and ground was adjusted to 3 volts and the voltage'of anode 4!] was approximately 200 volts and of cathode 4|, approximately volts. It was found in a particular test that, for an opening angle of 100 used for the calibration pattern angle, tuning casually, the incoming signal frequency could be set within .2 volt of the zero voltage cross-over point of the discriminator, While tuning with a reasonable amount of care, a signal frequency could be set within .1 volt or'less of the discriminator cross-over voltage.

While I have shown my indicator circuit as used in conjunction with a discriminator of 'a frequency modulation receiver, itmay be used for'other purposes such as, for example, to obtain zero voltage indications in a bridge circuit. It is apparent, therefore, that it may be used wherever it is desired to compare a voltage being measured with a standard voltage and it will :be understood that while I have shown aparticular embodiment of my invention, I do not wish to be limited thereto since various modifications may be made, and I contemplate by the appended claims to coverany such modifications as fall within the true spirit and scope of my cator means capable of producinga pair of indeanode, a cathode, and a pair of controlelectrodes, said anode being connected to one of said terminals and said cathode and one of said electrodes being connected to said intermediate contact, thereby to provide a first shadow angle of substantially constant value in said cathode ray means, and means supplying between said other electrode and said anode a potential Varying in accordance with a voltage to be calibrated, thereby to provide a second shadow angle in said cathode ray means for comparison with said first shadow angle.

2. A voltage indicator comprising a resistor having a pair of terminals and a pair of intermediate contacts, a source of unidirectional potential connected acrosssaid terminals, cathode ray indicator means capable of producing a pair of independent electrical shadows and compris ing an anode, a cathode, and a pair of control electrodes, said anode being connected to one of said terminals and said cathode and one of said electrodes being connected to one of said intermediate contacts, thereby to provide a first shadow angle of substantially constant value in said cathode ray means, and means supplying between said other electrode and said anode a potential varying in accordance with a voltage to be calibrated, thereby to provide a second shadow angle in said cathode ray means for comparison with said first shadow angle, and means including the other of said intermediate contacts for adjusting said variable potential to calibrate said cathode ray indicator means.

3, In a receiver for frequency modulated radio waves, the combination of a balanced frequency discriminator, said discriminator having an output circuit, and means for indicating the tuning of said receiver, said means comprising cathode ray indicator means capable of producing a pair of independent shadows and comprising an anode, a cathode and a pair of control electrodes, means connected to said output circuit to supply between said anode and one of said electrodes 'a unidirectional potential varying in accordance with the voltage of said output circuit, thereby to produce in said cathode ray means a variable shadow angle indicating the tuning condition of said receiver, and means to supply between said anode and the other of said electrodes a unidirectional potential of substantially constant value thereby to produce in said cathode ray means a second shadow angle of substantially constant width, the value of said substantially constant unidirectional potential and the width of said constant shadow angle being substantially the same as the value of said variable potential and the width of said variable shadow angle, respectively, at a desired tuning condition of said receiver.

4. A tuning indicator for a frequency modulation receiving apparatus comprising a balanced discriminator having an output circuit, a cathode ray indicating device capable of producing a pair of independent shadows and comprising a cathode, an anode and a pair of control electrodes, means for supplying between said anode and said cathode a substantially constant unidirectional potential, means for fixing the potential of one of said control electrodes with respect to -said cathode thereby to provide in said cathode ray device a first shadow angle of substantially constant width, a resistor connected between said anode and the other of said control'electrodes,

and means connected to said output circuit for supplyingto said resistor a current varying in accordance with the voltage of said output circuit thereby to control the potential of said second control electrode with respect to said cathode and to produce in said cathode ray device a second shadow angle of variable width indicating the tuning condition of said receiving apparatus.

5. A tuning indicator for a frequency modulation "receiving apparatus comprising a balanced discriminator having an output circuit, a cathode ray indicating device capable of producing a pair of independent shadows and comprising a cathode, an anode and a pair of control electrodes, means for supplying between said anode and said cathode a substantially constant unidirectional potential, means for fixing the potential of one of said control electrodes with respect to said cathode thereby to provide in said cathode ray device a first shadow angle of substantially constant width, a resistor connected between said anode and the other of said control electrodes, means connected to said output circuit for supplying to said resistor a current varying in accordance with the voltage of said output circuit thereby to control the potential of said second control electrode with respect to said cathode and to produce in said cathode ray device a second shadow angle of variable width indicating the tuning condition of said receiving apparatus, and means for adjusting said current to render said shadow angles substantially equal at a desired tuning condition of said receiving apparatus.

6. A tuning indicator for a frequency modulation receiving apparatus comprising a balanced discriminator having an output circuit, a cathode ray indicating device capable of producing a pair of independent shadows and comprising a cathode, an anode and a pair of control electrodes, a source of electric current supply connected to maintain between said anode and said cathode a substantially constant potential and to fix the potential of one of said control electrodes with respect to said cathode thereby to produce in said cathode ray device a first shadow angle of substantially constant width, a resistor connected between saidanode and the other of said control electrodes, an electric discharge device connected to conduct current from said source through said resistor, and means connected to said output circuit for controlling the intensity of said current in accordance with the voltage of said output circuit thereby to control the potential of said other control electrode with respect to said cathode and to produce in said cathode ray device a second shadow angle of variable width indicating the tuning condition of said receiving apparatus.

7. A tuning indicator for a frequency modulation receiving apparatus comprising a balanced discriminator having an output circuit, a cathode ray indicating device capable'of producing a pair of independent shadows and comprising a cathode, an anode and a pair of control electrodes, a source of electric current supply connected to maintain between said anode and said cathode a substantially constantv potential and to fix the potential of one of said control electrodes with respect to said cathode thereby to produce in said cathode ray device a first shadow angle of substantially constant width, a resistor connected between said anode and the other of said control electrodes, an. electric discharge device connected to conduct current from said source through said resistor, means connected to said output circuit for controlling the intensity of said current in accordance with the voltage of said output circuit thereby to control the potential of the other of said control electrodes with respect to said cathode and to produce in said cathode ray device a second shadow angle of variable width indicating the tuning condition of said receiving shadow angles of substantially equal width when the voltage of said output circuit is substantially JOHN E. MAYNARD. 

